Until I have a chance to see the prospectus I will be guessing a little based on securitisations i have worked on. I'm not sure what your familiarity is with ABS securitisation so forgive me if i am starting from a position that is too simplistic.
At the start of the securitisation the Noteholders are entitled to interest at the rate at which they purchased the Notes, and to all principal payments due under the $608m of leases up to the initial balance of the notes ($546m). Monthly lease payments are split into a principal and interest component.
There are generally two waterfalls (priorities of payment) in a securitisation, one that takes the interest income component received from all the monthly lease payments and attributes it to the expense payments due in the month; and one that takes the principal received and allocates it to repayment of the principal of the notes. (P13 of the Moody's pre-sale shows only one waterfall, I'm not sure if this is Moody's attempt to aggregate them or if this securitisation has a different structure to ones I have worked on. I will find out when the prospectus is released at closing)
The excess spread (called remaining fund in the Moody's Presale) is what is left after paying the expenses (trustee fees, swap costs and interest on the notes). Depending on the specific structure of the securitisation, there is likely to be a requirement for the interest income to reimburse the shortfall in principal received in the month for any defaults in the month. This is where any RV shortfall on a specific loan impacts Tesla - If the lease does not receive the contractual RV after sale of the vehicle then interest income from all the other loans can be used to top-up the loss of the RV. So technically Tesla does limit the shortfall RV risk by securitising, however there are so many safeguards to the bondholders in the deal (10% OC, reserve funds, interest income to cover shortfalls) that this downside would only be limited under Great Depression like circumstances. We should hope that Tesla never ends up realising it's downside risk limits as any failure to repay bonds is basically a death knell for a securitisation program. I should also note here that if the vehicle is sold for more than the contractual RV amount at the end of the lease, the contract (an therefore the securitisation) is not entitled to the extra cash.
Ultimately Tesla should be in the same position financially regardless of whether they securitise these assets vs if they could borrow the same amount of money at the same costs for the same time. All interest and principal received from the leases in excess of what is owed to the Noteholders will eventually find its way back to Tesla, although there will be some timing differences on receipt of the excess funds.
The spread is stated in this Wall Street Journal article states that the Class A notes sold for Libor + 0.3% and the Class E notes sold for Libor + 2.65%. The weighted average rate will probably Libor +~0.6%-0.8% (the article doesn't break down the cost for each tranche of notes). Another thing to mention is as the leases pay down, the principal is applied to the highest rated (and cheapest) notes first, which means the average interest rate on the notes goes up over time.
I would expect the WL to have broadly the same structure as the term securitisation in terms of income from all leases covering losses on individual leases and Tesla receiving any excess spread.
One thing I don't know is what the required credit enhancement is of the WLs. In the term deal Tesla has to fund the OC portion with it's own money, which is 10.2% of the balance of the loans. If the WL required Tesla to fund e.g. 20% of the leases sold into the WL, then there is a net 10% of the balance of the loans released to Tesla upon closing.
